Method of controlling fractionator or stripper temperature by controlling reboiled vapor flow



April 29, 1969 w, c, HART 3,441,483

METHOD OF CONTROLLING FRACTIONATOR 0R STRIPPER TEMPERATURE BY CONTROLLING REBOILED VAPOR FLO Filed June 7, 1967 |9 lHCL 24 N-PARAFFIN ABSORBENT 12 STRIPPER ZFEED 32 as as BENZENE 3C? l HOT l LC 9? 44 FRC 54 J8 72 l l A y ea- 4 HOT on. MAKE -UP r 58 i f\ INVENTOR.

w. c. HART BENZENE BY 7;

ATTORNEYS United States Patent 3,441,483 METHOD OF CONTROLLING FRACTIONATOR OR STRIPPER TEMPERATURE BY CONTROLLING REBOILED VAPOR FLOW Walter C. Hart, Bartlesville, 0kla., assignor to Phillips Petroleum Company, a corporation of Delaware Filed June 7, 1967, Ser. No. 644,299 Int. Cl. B01d 3/42 U.S. Cl. 203--2 8 Claims ABSTRACT OF THE DISCLOSURE In controlling temperature in a fractionation column, a liquid bottoms stream from the column is passed to a reboiler where it is heated by indirect heat exchange with a heating fluid so as to vaporize a substantial portion of the bottom stream. A portion of the vaporized stream is passed into the bottom of the column as reboil fluid and the remaining portion is separately recovered. Control of column temperature is efiected by sensing the temperature on a selected column tray and regulating the flow rate of one of the vapor streams in response to the sensed temperature to maintain a relatively uniform temperature on said tray.

This invention relates to a method for controlling the temperature in a fractionator or stripper column.

My copending application, Ser. No. 422,856, filed Dec. 31, 1964 and issued June 4, 1968 as U.S. Patent No. 3,386,891, discloses and claims a process or method of control for a stripper or fractionator which holds a set temperature on the control tray by regulating the flow of heating fluid to the reboiler heating coil. This method was workable but the response of the system was slower than desired. The reboiler also is used to flash excess benzene out of the system to be recycled to the 'alkylation process from which it originated. The reboiler, therefore, is large in size compared to the stripper or fractionator column. As a result, it has a large heat capacity and a very slow response to changes in reboiling requirements. This slow response is aided by the low temperature heating fluid used for reboiler heating because the AT between the hot oil and the reboiler is comparatively small, operating at a 6 F. approach.

The instant invention is concerned with an improved method of controlling the column temperature which eliminates the foregoing problems.

Accordingly, it is an object of the invention to provide an improved method of controlling the temperature in a fractionator such as a stripper column. Another object is to provide a method of controlling temperature in such a fractionator which is more responsive and has smaller time lag than in conventional control systems. Other objects of the invention will become apparent to one skilled in the art upon consideration of the accompanying disclosure.

A broad aspect of the invention comprises flashing a substantial portion of a bottoms stream from a fractionator column in the reboiler of the column, passing a portion of the flashed stream into the bottom of the column as reboil fluid, passing the remaining portion of the flashed stream to recovery, sensing the temperature on a selected tray in the column and regulating either the flow of vapor into the column bottom section or the flow of vapor to recovery in response to the sensed temperature to maintain a relatively constant preselected temperature on the tray. When controlling the flow of vapor to the bottom of the column in response to the sensed temperature, the flow is varied directly in response to the temperature whereas, when the control is on the vapor stream passing to recovery, the flow rate is controlled inversely to the temperature. In other words, the flow rate of reboil fluid is increased as the sensed temperature falls below the predetermined value and vice versa. In controlling the flow of vapor to recovery, the rate of flow is increased as the sensed temperature rises above the selected temperature and vice versa. The control of the flow of liquid bottoms product from the reboiler is etfected with a liquid level control operatively connected with a flow rate controller on a motor valve in the efiluent product line. Temperature of the liquid in the reboiler is maintained relatively constant by regulating the rate of flow of heating fluid thereto.

A more complete understanding of the invention may be had by reference to the accompanying drawing which diagrammatically illustrates a preferred embodiment of the invention.

Referring to the drawing, a fractionator 10 is provided with an absorption or reflux section 12 and with a number of trays in section 14 for effecting vapor-liquid contacting. Feed line 16 connects with the column for introducing feed to the trays at a uniform rate controlled by flow controller 17 operatively connected with motor valve 18. An overhead vapor line 19 connects with the top of the column and an absorption liquid feed line 20 containing a motor valve 22 operated by a flow rate controller 24, connects with the upper section of the column above the absorption section 12. A bottoms efiluent line 26 connects with the bottom of column 10 and with reboiler 28. Reboiler vapor effluent line 30 connects with line 32 leading into the bottom section of the column and with line 34 which passes excess reboil fluid to recovery. Line 34 is provided with a cooler or condenser 36, a motor valve 38, and a flow rate controller 40 operatively connected with line 34 and with motor valve 38. A temperature controller 42 is sensitive to the temperature of liquid on a selected tray in column 12 and is in operatively control of flow rate controller 40.

Reboiler 28 is provided with a heat exchange tube bundle 44, the inlet of which is connected with a supply of hot oil in tank 46 by means of line 48. The outlet of tube bundle 44 is connected with tank 46 by means of line 50. Pump 52 circulates hot oil thru the system, including tube bundle 44, the flow thru which is controlled by temperature controller 54, which is sensitive to the temperature of liquid bottoms in reboiler 28 and in control of motor valve 56 in line 48. Hot oil makeup is supplied from line 58 which connects with a heater, not shown, and is provided with a motor valve 60 operated by a temperature controller 62 which is sensitive to the oil temperature in line 48. In this manner, the oil temperature in the oil circulating system is maintained at a selected level. Line 64 returns hot oil to the heater from tank 46 as hot oil makeup is added.

Efiluent bottoms product is withdrawn from reboiler 28 thru line 66 from the end section of the reboiler downstream of overflow baflle 68. This baflle maintains a liquid level in the upstream section of the reboiler sufficient to maintain tube bundle 44 covered with liquid. Withdrawal of liquid bottoms products thru line 66 is controlled by level controller 70 which maintains a selected level in reboiler 28 by controlling flow rate controller 72 which is in operative control of motor valve 74.

The method of controlling fractionator temperature in accordance with the invention is applicable to the fractionation or stripping of any multi-component liquid stream, the components of which have substantially different boiling points. Thus, while the specific description of the process herein is directed to a specific application of the invention, it is not so limited as will be understood by one skilled in the art.

A specific application of the invention is in the stripping of HCl from an alkylate reaction mixture comprising HCl, benzene, n-paraffins, and phenylalkanes. The process of producing this alkylate mixture is fully disclosed in 3 4 my aforesaid patent application. The alkylate reaction of the column. By the method of the invention, a temmixture from the process is fed thru line 16 under the perature of 195 F.about 1 F. was maintained on control of fiow controller 17 and motor valve 18 at a the sixth tray. This novel temperature control provides selected constant rate into fractionator or stripper 12 much smoother column operation than was heretofore possible. Controlling the temperature of the oil circulating with heat being supplied to the liquid bottoms in the thru the tube bundle in the reboiler at a constant level column by benzene vapor from line 32. HCl which has the lowest boiling point of any of the feed components is and maintaining a uniform reboiler temperature, facilivaporize-d along with a minor proportion of the benzene tates the temperature control in the column. in the feed and ascends the stripping section of the column Certain modifications of the invention will become where it is contacted with an absorption stream of normal apparent to those skilled in the art and the illustrative paraflins recovered from the feed and introduced to the details disclosed are not to be construed as imposing upper end of the column thru line 20 at a fixed rate under unnecessary limitations on the invention. the control of fiow rate controller 24 and valve 22. The I claim: n-paraifin absorbent removes benzene from the ascending 1, A method of heating a liquid in the bottom of a stream of HCl. The n-paraffin absorbent and the HCl 1 fraetionati g one whi h comprises the steps of: vapors make good contact in the absorption section of (a) ing a bottoms tream from said zone into a rethe tower which is provided with vapor liquid contacting boiler in i dire t heat exchange with a heating fluid material. so as to vaporize a substantial portion of said bot- Bottoms liquid is passed thru line 26 into reboiler 28 toms stream and leave a substantial liquid portion; where it is heated by indirect heat exchange with oil in 20 (b) removing said liquid portion of step (a) a b tube bundle 44 so as to vaporize a substantial proportion toms product; of the benzene in the bottoms stream, the benzene passing passing a first substantial portion of the vaporized overhead in vapor form thru line 30 from which a portion portion of step (a) into the bottom of said fractionapasses into the column bottoms thru line 32 and the retion zone as reboil fluid; maining portion passes thru condenser 36 and line 34 to (d) ithdr ing a e o d remaini ortion of said recycle to the alkylation process. vaporized portion of step (a) as product;

In the arrangement shown in the drawing, a selected Sensing the temperature on a selected y in Said predetermined temperature is maintained on an intertraetiorlatihg Zone; and mediate tray in the column by means of temperature conregulating the 11ow rate of one of Said first P troller 42 which senses the temperature of liquid on the tion and Second Portion of Steps and so as tray and operates flow rate controller in known manto maintain a relatively Constant Selected p ner to control the flow thru valve 38, increasing the flow ture in P rate as the sensed temperature tends to rise from the 2. The process of claim 1 wherein step (f) comprises selected level and decreasing the flow rate as the sensed increasing the flow rate of said second portion when the temperature tends to fall from the selected level. In this sensed temperature rises from the selected temperature manner, the flow rate of benzene to recovery is varied and decreasing the flow rate thereof when the sensed inversely as the temperature varies. In the event the flow temperature lowers from the selected temperature on said control is placed on the flow of benzene in line 32, the tray.

3. The process of claim 1 wherein step (f) comprises decreasing the flow rate of said first portion when the flow rate is varied directly as the temperature varies.

Flow rates and conditions of operation are now pre- 40 sented as illustrative of the invention and are not to be sensed temperature rises from the selected temperature construed as unnecessarily limiting the same. and increasing the flow rate thereof when the sensed In operating in accordance with the invention, in aptemperature lowers from the selected temperature on said paratus substantially as shown in the drawing, in sepatray.

rating HCl from an alkylate reaction mixture, the oper- 45 4. The process of claim 1 wherein a feed stream comating conditions and stream contents obtained are set prising HCl, benzene, n-paraffin, and phenylalkanes is forth below. introduced to said fractionation zone to fractionate same TABLE Pounds per hour Reboil Benzene Line No Feed (16) O.I-I.V. (19) Reflux (20) vapors (32) recycle (34) K1. (66) HCI Benzene Parathns Phenylalkanes. Diphenylalkanes- A101 Total 59, 042 1, 182 2, 782 21, 227 17, 584 43, 05s

The temperature of the feed in line 16 is about 95 F. into an overhead stream comprising essentially HCl, and and the total flow is about 4,820 barrels per day. The a bottoms stream comprising essentially the remaining temperature in the kettle is maintained at about 244 F. components of said stream; said bottoms stream is the feed at a pressure of 20 p.s.i.a. (pounds per square inch abstream to step (a); the vaporized portion of said bottoms solute). The temperature in the top of the column is stream in step (a) comprises essentially benzene; and the 50 F. and the pressure is 15 p.s.i.a., the rate of flow of bottoms product of step (b) comprises essentially n-paraf- HCl vapor being about 8,410 s.c.f.h. Parafiin absorbent fin, phenylalkanes, and remaining liquid benzene. enters the P of the e h at e temperature f 5. The process of claim 4 wherein operating conditions The bottoms Product lrrhrre 66 15 t about 244 and in said ifractionating zone comprise a top temperature of 20 1 the rate of Wlthdrawal helrlg about 41 about 50 F. at a pressure of about 15 p.s.i.a. and a botr P y- The flow rate of benzene Vapor lrl hhe 32 70 toms temperature of about 244 F. at a pressure of about Is about 1,652 barrels per day at a temperature of about 20 i and a p e of about 20 P- The rate of 6. The process of claim 1 wherein the removal of said herlZehe absorbent 15 about 1,368 barrels P liquid portion in step (b) is controlled by sensing the liq- The system is operated with the temperature control uid level of liquid in said reboiler and controlling the rate on the sixth tray, there being twelve trays in section 14 of removal to maintain a selected liquid level therein.

5 6 7. The process of claim 1 wherein the temperature of 2,886,616 5/ 1959 Mertz et a1. 196-132 the liquid bottoms in said reboiler is sensed and the flow 2,917,437 12/ 1959 Kleiss et a1. 2032 rate of the heating fluid in step (a) is controlled in re- 3,208,230 9/1965 Fourroux 196132 sponse to the sensed temperature to maintain a selected 3,223,749 12/1965 Van Pool et a1 196-132 relatively constant reboiler temperature. 3,249,518 5/1966 Vautrain et a1. 202160 8. The process of claim 7 wherein the temperature of 5 3,249,519 5/1966 Cabbage et a1. 2032 said heating fluid passing into said reboiler is maintained 3,301,778 1/ 1967 Cabbage 2032 substantially constant.

WILBUR L. BASCOMB, JR., Primary Examiner.

References Cited 10 US. Cl. X.R5

UNITED STATES PATENTS 203 9s, 100; 202160; 260-671, 674; 23-454; 2,545,671 3/1951 Passino. 196-132 

